In many imaging applications excessive film curl can cause serious difficulties with film transport and handling and it is, therefore, important to reduce the core-set propensity of the image-bearing film to meet system specifications. Over the years many approaches have been taken to reduce curl in photographic films, particularly that associated with core-set. These can be generally grouped in terms of four distinct mechanisms: (1) Physical aging, (2) Inherent curl, (3) Ironing, and (4) Reverse winding. Each of these mechanisms is applicable for certain types of films and selection of one over the other depends on the particular circumstances of the problem at hand. Following is a brief summary of these general approaches:
Physical Aging
This method is practiced by heating the finished film (usually but not only) in a wound state to relatively high temperatures (typically 10 to 40.degree. C. below the glass transition) for relatively long times (typically &gt;1 day) in order to lower the propensity of the film to take up curl in subsequent winding steps. This high temperature annealing treatment changes the relaxation characteristics of the constituent material (an aged film relaxes slower than a fresh film) and is especially useful when the final winding diameter of the film is much smaller than the diameter during annealing. U.S. Pat. Nos. 5,254,445, 5,629,141 5,585,229 and 4,141,735 are examples based on this general principle, and are incorporated by reference herein.
Inherent Curl
During the manufacture of film support it is possible to induce curl in a given direction by differentially (asymmetrically) heating the support during the stretching operation (preferably during the drafting step), i.e., by inducing a temperature gradient of about 10-15.degree. C. across the thickness of the film as it is being stretched above the glass transition temperature (Tg). If this inherent curl is in a direction opposite to the expected core-set curl, it will offset, to some extent, the curl induced during winding and will yield lower final curl. This method requires significant modifications of the film manufacturing process and fine-tuning of the stretching temperature during the drafting and/or tentering steps. U.S. Pat. Nos. 4,892,689 and 4,994,214 are examples of this approach and are herein incorporated by reference. The latter combines the inherent curl approach with physical aging, i.e., after an inherent curl is induced in the film it is annealed under typical aging conditions.
Ironing
By heating relatively short and narrow film sections to high temperatures, in the vicinity of Tg, it is possible to remove curl induced by core-set. This method requires some tension as the film is transported through the ironing device and the film must be either flat or slightly curved in a direction opposite of the expected core-set curl. Residence times for this heating method are relatively short, of the order of minutes or less. However, this method is not ideally suited for treating wide and long production rolls because of difficulty of controlling temperature uniformity and the possibility of distorting and scratching the film and damaging the coated emulsion layers within the ironing device. This general approach is taken in U.S. Pat. Nos. 3,916,022, 4,808,363, 4,851,174 and 5,549,864 herein incorporated by reference.
Reverse Winding
By winding the film in the opposite direction of its induced core-set curl, the curl level can be reduced. This approach can be practiced at any temperature but the rate of curl change depends on the temperatures at which the film is stored and rewound and it may require very long time to achieve a meaningful reduction in curl at ambient conditions. U.S. Pat. No. 3,806,574 falls under this general category although it does not provide any clear guidelines as to the conditions of the rewinding step necessary to achieve partial or complete erasure of curl.
In the present invention the core-set curl propensity of the film is modified by laminating the film support with an appropriate polymeric layer and placing it on the convex (`out`) side with respect to the expected winding direction. The laminated layer can be applied by co-extrusion, co-casting or by coating using conventional methods and its thickness must exceed some critical limit. This approach eliminates the need to do on-line or off-line decurling as noted above and it may also eliminate the need to anneal (`age`) the support or at least reduce the severity of the annealing treatment (e.g., lower annealing temperatures, shorter annealing times).